This invention concerns a process for producing .beta.-form Si.sub.3 N.sub.3 (hereinafter simply referred to as .beta.-Si.sub.3 N.sub.4) from amorphous or .alpha.-form Si.sub.3 N.sub.4 (hereinafter simply referred to as .alpha.-Si.sub.3 N.sub.4), as well as a process for producing a Si.sub.3 N.sub.4 article containing .beta.-Si.sub.3 N.sub.4.
Amorphous Si.sub.3 N.sub.4 and crystalline .alpha.- or .beta.-Si.sub.3 N.sub.4 are known. Among the three types of Si.sub.3 N.sub.4, .beta.-Si.sub.3 N.sub.4 is considered to have the highest corrosion resistance against molten silicon or the like.
Conventionally in order to prepare .beta.-Si.sub.3 N.sub.4 amorphous or .alpha.-Si.sub.3 N.sub.4 having low purity and containing metal oxides such as Y.sub.2 O.sub.3 or metal nitrides such as TiN in amounts of about 5 to 20% by weight has been heat treated at a temperature from 1500 to 1700.degree. C. Accordingly, .beta.-Si.sub.3 N.sub.4 produced by the conventional process contains a relatively large amount of impurities. Reaction sintered Si.sub.3 N.sub.4 is also known as .beta.-Si.sub.3 N.sub.4 having a relatively high chemical purity, but the reaction sintered Si.sub.3 N.sub.4 also contains not less than 0.5% by weight of impurities such as A1, C and O (in total), due to the impurities in the starting Si.sub.3 N.sub.4 material or source Si.sub.3 N.sub.4 material. On the other hand, the CVD (Chemical Vapor Deposition) process is also known as a process for producing Si.sub.3 N.sub.4 having high purity. However, the Si.sub.3 N.sub.4 having high purity produced up until now by the CVD process is amorphous or .alpha.-form Si.sub.3 N.sub.4.
No process is yet known for changing the Si.sub.3 N.sub.4 produced by the CVD process into .beta.-form while keeping the purity of the Si.sub.3 N.sub.4 high.